This invention relates to a catalytic alkylation process. It particularly relates to an improved process for the separation of the hydrocarbon and acid components present in the effluent from a catalytic alkylation zone. It specifically relates to an improvement to eliminate the overhead vapor apparatus of the HF regenerator, including the overhead condenser, overhead receiver, and overhead pump.
It is well known in the prior art that catalytic alkylation using hydrofluoric acid or sulfuric acid as the catalyst has become an important chemical tool for preparing alkylated hydrocarbons and derivatives thereof. The commercial and industrial demand for these products is exemplified with the demand for isoparaffin hydrocarbons and alkyl-substituted benzenes of gasoline boiling range and with the demand for alkyl-substituted aromatics suitable for conversion to surfactants, e.g., detergents, wetting agents, etc. The prior art process of alkylation generally is effected by contacting an isoparaffin hydrocarbon feed stock with an olefin hydrocarbon in the presence of a catalyst such as hydrofluoric acid in a suitable reaction vessel for conducting chemical reactions.
In practice, there have been numerous process schemes advanced by the prior art for accomplishing the alkylation reaction, but it is extremely difficult to achieve a process scheme which embodies all of the desirable features of a completely optimum reaction. Optimizing the alkylation reaction is complicated by the fact that the alkylation reaction if not carried out properly has many side reactions, such as polymerization, which destroys the effectiveness of the reaction and inhibits the production of commercial quantities of desired alkylate. Additionally, the reaction, in order to be carried out commercially, requires a tremendous amount of auxiliary equipment for the recovery of the alkylate product, for the regeneration and reuse of the excess catalyst, and for the recovery and reuse of the excess reactants which have passed through the reaction system.
The catalytic alkylation process to which the present invention is applicable consists of a process in which a mixture of hydrocarbons containing isoparaffins such as isobutane, isopentane, and the like, and olefins such as propylene, butenes, isobutenes, and the like, are mixed intimately in the presence of a strong acid catalyst, such as hydrofluoric acid or sulfuric acid at generally room temperatures or lower for sufficient time to complete the reaction. The effluent from the reaction zone contains saturated isoparaffin hydrocarbons of higher molecular weight or boiling point than the isoparaffin in the original mixture. For convenience, these higher molecular weight isoparaffin hydrocarbons which comprise the reaction product from the alkylation zone are called "alkylate." Isobutane has been used almost exclusively because of its reactivity and availability to produce high quality alkylate product. In similar manner, among the olefins, butenes and propylenes have been used satisfactorily. In some cases it is desirable to use solely propylene or butene as the olefin reactant.
As is typical in most commercial chemical plants, the reaction between the isoparaffin hydrocarbon and the olefin hydrocarbon is performed with an excess of isoparaffin in the reaction zone. Accordingly, there is a large excess of the isoparaffin hydrocarbon remaining in the effluent from the reaction zone. Additionally, there is a significant quantity of C.sub.3 hydrocarbons which pass through the system, and for economy sake, must be recovered in as high yield as possible. In similar manner, it is desirable to recover for reuse the isoparaffin reactant in as high yield as possible, which is accomplished in an isoparaffin stripper, or, more specifically when isobutane is the isoparaffin, an isostripper.
In my prior application, U.S. Ser. No. 533,421, now U.S. Pat. No. 3,956,416, I disclosed an HF alkylation process wherein the reactor effluent was separated into a hydrocarbon phase and an acid phase, and the hydrocarbon phase passed through a train of two fractionators. The first fractionator was an isobutane stripper which produced a bottoms fraction of pure alkylate, a n-C.sub.4 side-cut vapor fraction, a recycle isobutane side-cut vapor fraction, and an overhead fraction of isobutane and lighter materials. The overhead fraction was charged to a depropanizer, to remove propane from isobutane. The advantage of operating with two fractionators is that the overall pressure of the system is much lower than if this same fractionation were attempted in a single column.
It is also know to operate fractionation columns so that a liquid isobutane recycle stream is obtained. The fractionation scheme disclosed in my copending application mentioned above would be most suitable for obtention of a vapor phase isobutane recycle stream. In U.S. Pat. No. 3,371,032 (Class 208-351), the teachings of which are incorporated by reference, a fractionator is disclosed which will permit recovery from the fractionator of an isobutane-rich stream which is in the liquid phase. A relatively pure propane stream is recovered overhead in such a system. When a single fractionator is used in this fashion, it will be called a "main fractionator." This terminology is in contrast to the two fractionator system of my prior patent, which requires an isobutane stripper and a separate depropanizer.
The process of the present invention should not be confused with that disclosed in U.S. Pat. No. 3,763,265 (Class 260-683.42). In that patent a fractionator is disclosed which provides for the addition of makeup and/or re-run HF acid to the upper portion of a fractionator, with a side-out recycle isobutane stream being withdrawn from a lower portion of the fractionator. This is merely a conventional fractionator combined with an extractive distillation zone. The conventional fractionator operates to produce vapors containing essentially propane and ethylfluoride and a liquid-containing propane and alkylfluoride, and subjecting these vapors and liquids to extractive distillation with hydrogen fluoride. Liquid hydrogen fluoride is introduced to the top of the extractive distillation zone to absorb ethylfluorides so that they can be returned, in solution in HF acid, to the alkylation zone. The process works better with higher purity HF acid, so the patentee provides for the introduction of makeup HF, or re-run HF, or HF which is recovered from the overhead of a conventional depropanizer. Such an operation, wherein extractive distillation is practiced using a liquid HF acid phase to absorb ethylfluorides, and wherein a recycle isobutane stream is withdrawn from a fractionator does not come within the scope of the present invention.
Another problem associated with the operation disclosed in U.S. Pat. No. 3,763,265 is that the fractionation of the liquid HF added to the top of the fractionator is relatively inefficient, while contributing substantially to the heating load of the column.
The fractionation of this liquid HF fraction is inefficient because a lot of the liquid will be vaporized and go overhead immediately after entering the fractionator. Addition of hydrocarbon reflux, via line 30, to a point above the inlet point of the HF acid line 24, will absorb some of the vaporized HF acid added, but there will still be a very large amount of HF acid vapor going overhead in this fractionator. There is a significant loss of entropy of the system in adding a relatively pure HF acid stream via line 24, and then adding above this stream a relatively pure hydrocarbon fraction via line 30. Quite a lot of work was accomplished in obtaining these two streams in a relatively pure state, and quite a lot of work will be lost in mixing them together at that point in the fractionator with no benefit obtained thereby.
The heating load of the column will be increased because the HF acid added via line 24 must either be vaporized and removed from the system via line 9, or must be eventually recovered as a hotter liquid via line 22. The net effect will be to increase the amount of heat which must be added to the bottom of the column.
To recover propane product in the overhead 9 while maintaining a liquid HF phase 22 would require substantial reflux thru line 30 and line 20. HF requires substantially more energy for vaporization compared to the C.sub.2 -C.sub.4 hydrocarbons on the same unit base. More reflux and more heat input to subsequently vaporize this reflux are necessary to maintain this operation.
In contrast, addition of a vapor phase HF hydrocarbon mixture fraction to this column will not increase the heating requirement. As a matter of fact, since these vapors are superheated, and consist of HF and C.sub.3 or iC.sub.4, the heating requirement will decrease.
If propane is used as a stripping medium in the regenerator, both C.sub.3 and HF are overhead vapor products, at a temperature lower than their inlet to the column. Thus it will reduce the heating requirement.
If iC.sub.4 is used as the stripping medium the iC.sub.4 portion goes with the iC.sub.4 side cut (liquid) and HF as overhead vapor. Again for the same fractionation it will reduce the reboiler load.
Addition of an overhead vapor or liquid stream from an HF acid regenerator to this fractionator at some point several trays below the top of the fractionator would also increase the efficiency of the operation. The efficiency would be increased because the overhead fraction from an HF acid regenerator can be very cheaply fractionated in the main fractionator, and fractionation of this stream is very important.
Presuming that e.g., isobutane is used as the stripping medium in the HF acid regenerator, to use the isobutane portion of the regenerator overhead to benefit the alkylation reaction would require a condenser and a phase separator. The liquid isobutane phase then can be recycled to the alkylation reactor to increase the iC.sub.4 /olefin ratio which will improve the alkylate quality. Even then the benefit is partial, as the liquid iC.sub. 4 from the phase separator contains dissolved HF which, when it comes in contact with the olefins prior to the reaction chamber, where the acid concentration is substantially low, gives poor reaction products. Thus any contact of HF with olefin outside the alkylation reactor must be avoided or minimized as much as possible.
A discussion of this problem, and another means of solving it is disclosed in U.S. Pat. No. 3,879,488, the teachings of which are incorporated herein by reference. For the present discussion, it is enough to note that for the isobutane stripping agent used in the HF acid regenerator to be reused again in the HF alkylation zone, it must be subjected to fractionation to permit recovery of an isobutane fraction which is not saturated with HF acid.
If propane is used as a stripping medium, introducing the regenerator overhead vapors to the main fractionator just below the reflux (2-5 trays) would reduce the heat load on the column, but all the propane and HF will be recovered as overhead vapor product. This scheme also saves the cost of an acid regenerator separator condenser receiver and transporting system.
Briefly restated, addition of condensed liquid derived form the overhead vapor of a conventional HF acid regenerator will increase the heat requirement of the fractionator receiving this material. Addition of either a vapor or liquid stream from the regenerator to the top of a fractionator will waste energy as this will not be the optimum feed point location.
Accordingly one skilled in the art would be reluctant to use the process disclosed in the U.S. Pat. No. 3,763,265, unless needed to overcome an extraordinary problem, namely the recovery of ethyl fluorides. Ethyl fluorides are not a significant problem in most alkylation processes, and refiners would be reluctant to pay the onerous cost of utilities involved with such a process unless they were encountering significant problems with ethyl fluoride accumulation or loss.
In these catalytic alkylation processes there is a need for periodic regeneration of the catalyst system. This was usually accomplished by taking a stream of at least a portion of the acid catalyst, e.g., hydrofluoric acid, and passing it to a regeneration column wherein the regenerated catalyst is stripped with a light hydrocarbon, for example, hot or superheated vaporous isobutane. The purpose of this regeneration is to remove from the catalyst impurities such as water and acid soluble oils which accumulate in the system. These oils are of a polymeric composition which is in equilibrium with the alkylate hydrocarbon and heavy tar produced in the alkylation reaction. As used in this specification, these impurities and/or contaminants in the catalyst phase are for convenience lumped together and characterized as being material boiling above the boiling point of hydrogen fluoride acid.
The prior art processes for regenerating liquid catalyst such as hydrofluoric acid catalyst usually involve distillation schemes which present problems both from a process standpoint and from an apparatus standpoint. For example, since it is an acid system, the presence of water will cause severe corrosion problems in the regeneration column and in any condensing means associated therewith. Expensive, high quality alloy metallurgy is provided in the various apparatus associated with the regenerator to reduce the rate of corrosion found in this system, and even so, frequency replacement of equipment is not unusual. In addition, sufficient heat must be applied to the catalyst stream in order to vaporize the catalyst for recovery as a purified product. However, in the vaporization of this catalyst stream there will remain a non-vaporized residue of heavy organic diluent which tends to foul the tubes of the heat inducing means. Another problem present in the prior art process is the difficulty of providing sufficient stripping media so that the acid losses to the tar residue are minimized. If sufficient stripping media is passed into the regeneration column so that no acid will remain in the bottom product, there is frequently entrained overhead an excessive portion of heavy organic diluent which then contaminates the vaporized catalyst stream thereby creating additional fouling problems in the lines and condensing means associated with the regeneration system.
In the prior art, several means have been used to eliminate the HF generator overhead system, which can be described as the overhead condenser, overhead receiver, and overhead pump, or to combine that system with the overhead system of another fractionation apparatus. Thus it is seen in U.S. Pat. No. 3,349,146 that the regenerator overhead system is combined with the overhead system of a fractionator which strips HF from propane. Also in the prior art, in an isobutane stripper system wherein isobutane recycle is withdrawn from the isobutane stripper system as condensed overhead vapor saturated with HF, the overhead vapors of the HF regenerator are introduced into the overhead vapor conduit of the isobutane stripper upstream of the overhead condenser, thereby eliminating the regenerator overhead system. However, in the modern isobutane stripper, recycle isobutane is withdrawn as a side-cut from the isobutane stripper, and all overhead hydrocarbon product is withdrawn as feed to subsequent fractionation, i.e., depropanization. When the modern isobutane stripper came into use, it was considered desirable to separate the overhead systems of the regenerator and isobutane stripper in an effort to reduce incremental capital and operating costs of the depropanization fractionation, which wre deemed greater than the incremental capital and operating costs of the separate regenerator overhead system.
Another prior art way of eliminating the overhead system of the HF regenerator is disclosed in U.S. Pat. No. 3,478,125 (Class 260-683.48), the teachings of which are incorporated by reference. In this patent, the overhead fraction, comprising HF acid and stripping vapors is returned to the settler or alternatively to the reaction zone. Such a system will eliminate the overhead system in a regenerator, but is not a complete solution. If the overhead fraction from the HF regenerator is added to the settler, the heat of condensation of this stream may be adsorbed by the acid phase, which is generally undesirable, as it results in a higher temperature in the reaction zone, or it may be adsorbed by the hydrocarbon phase. If the HF vapors are charged into the portion of the settler containing hydrocarbon liquid, there will be a significant increase in the amount of HF acid which enters the fractionator. Further the HF acid, and its accompanying stripping agent, will be condensed in the settler, only to be vaporized in the fractionator. The major point is that the isobutane needed for stripping is circulating through the fractionation zone without any benefit as recycle. These difficulties will not preclude use of such a system, but decrease somewhat the energy efficiency of an HF alkylation unit. It would be desirable if the vaporized HF acid and stripping vapor could be charged to a fractionator operating at conditions similar to those encountered in the HF regenerator. By matching the conditions in the HF regenerator to the fractionation means used to separate acid from stripping vapor, the net increase in entropy of the system is minimized, and hence utility cost are minimized.